1. Field of the Invention
The invention relates to a method and apparatus for continuously monitoring wind turbine blades in rotating wind turbine generators for the propagation of latent defects and breaking adhesive bonds.
2. Description of the Related Technology
Due to their large size and extensive surface area and complex shape, wind turbine blades are difficult to non-destructively inspect in the factory. Visual inspection does not see defects below the surface. Thermography inspection techniques are somewhat effective but can give false positives and false negatives due to variations in material thickness and surface emissivity. Angle beam ultrasonic techniques are very slow and may not work through thick carbon fiber spar caps. As a result, blades are installed on towers and put into service with a significant probability of latent manufacturing defects. Furthermore, composite blades are subject seasonal temperature variations and entrapped water can undergo freeze/thaw cycles causing internal damage. Cyclic forces of gravity and varying forces from the wind acting on the blades as they rotate can cause fatigue damage or the propagation of latent defects over time.
Detecting progressive damage and propagating defects in wind turbine blades in situ is difficult. Inspectors using sky cranes or rope access are expensive, time consuming and put personnel in a very dangerous working environment. While uptower, close access allows inspectors to visually detect blade trailing edge splits, cracks, lightning damage and blade erosion, such inspections are intermittent, expensive and subjective.
The blades of commercial wind turbines are often several hundred feet off the ground. Access to wind turbine blades in situ with portable instruments for nondestructive testing accordingly has conventionally required rope access or sky platforms and cranes. This is time-consuming and possibly dangerous if the appropriate safeguards are not followed or if there is an equipment failure. Blade and tower crawlers with nondestructive testing sensors for in situ inspection have been developed and tested, again with high cost implications, slow inspection rates and questionable effectiveness. Microwave and radar scanners, while effective for dielectric materials, do not work on critical areas such as spar caps, which are often manufactured with electrically conductive carbon fiber materials.
New utility scale wind turbine blade designs are typically fatigue tested to failure at special facilities to accommodate the large size, often 50 meters span or more. Frequently, sensors such as fiber Bragg strain gages and acoustic emission (AE) sensors are bonded to the structures to allow monitoring during the entire test. While the use of acoustic emission (AE) sensors and technology is highly effective for detecting and locating propagating defects during ground based fatigue testing, standard AE practice requires bonding sensors to the blade throughout its span and in critical areas. The range of Rayleigh waves propagating in fiberglass is limited and multiple sensors are required raising cost and power requirements. Retrofitting the fleet of existing blades on wind generators in situ is a prospect both expensive and extremely hazardous.
Electricity generators designed to extract energy from the wind are powered by rotating turbines designed as either vertical axis wind turbines (VAWT) or horizontal axis wind turbines (HAWT). Large industrial scale power turbines are generally of the HAWT design using composite air foil shaped blades to generate the rotational torque needed to drive the electrical generator. Current utility scale wind turbine blades may range from 9 m in length up to more than 50 m, with much larger blades being designed for offshore wind power generators. The application of this invention may achieve good results on blades of all lengths.
A need accordingly exists for a cost effective wind turbine blade structural health monitoring system, both for the aging existing fleet as well as new wind turbines. There is a particular need for a wind turbine blade nondestructive testing system that is capable of performing testing and monitoring from the ground, and that is capable of providing remote notification or alerts as to the existence of propagating defects.